1. Field of the Invention
The present invention relates to a process for preparing silaoxacycles having structural units in which silicon and oxygen atoms are bonded to one another via a CH2 group, and to novel silaoxacycles.
2. Description of the Related Art
Silaoxacycles in which silicon and oxygen atoms are bonded to one another via a CH2 group are excellent reagents for the preparation of (hydroxymethyl)-polysiloxanes by termination of silicone oils according to the following reaction equation:

Since the silaoxacycle used as the terminating reagent, being a cyclic compound, has no end groups or the like which have to be eliminated in the reaction, the reaction I is a smooth addition reaction without any condensation products which would have to be removed thereafter. The thus produced carbinol oil terminated with Si—CH2—OH groups is of excellent suitability for the preparation of “AA-BB” polymers, for example by reaction with diisocyanates, assuming that the termination is quantitative, since every Si—OH group which is not terminated with an Si—CH2—OH group is converted in the course of subsequent preparation of AA-BB polymers by means of diisocyanates to an Si—O—C(O)—NH— group, the Si—O bond of which constitutes a hydrolysis-sensitive cleavage site. The greater the purity of the silaoxacycle used, the smoother the termination.
The specialist literature describes various methods for preparation of silaoxacycles in which silicon and oxygen atoms are bonded to one another via a CH2 group.
For instance, the preparation of 2,2,5,5-tetramethyl-1,4-dioxa-2,5-disilacyclohexane by heating of 1,3-bis-(hydroxymethyl)-1,1,3,3-tetramethyldisiloxane over calcium oxide has been described in U.S. Pat. No. 2,898,346 and Journal of Organic Chemistry 1960, vol. 25, p. 1637-1640. However, this process gives the product only in a 40-60% yield, requires the use of calcium oxide in an amount of about one quarter of the reaction mass, and gives an impure product, recognisable by the broad boiling range of the product fraction and by the elemental analysis reported, which has distinct deviations from the theoretical values. The poor purity of the product thus prepared is confirmed by Chemische Berichte 1966, vol. 99, p. 1368-1383 (see footnote 10 therein on p. 1373). Chemische Berichte 1966, vol. 99, p. 1368-1383 describes a process for preparing 2,2,5,5-tetramethyl-1,4-dioxa-2,5-disilacyclohexane by converting (acetoxy-methyl)ethoxydimethylsilane by heating it with a large excess of methanol in the presence of p-toluenesulfonic acid (p-TsOH) to give ethoxy(hydroxymethyl)dimethylsilane, neutralizing the primary product and then distilling gradually with elimination of ethanol (reaction II):

This process, however, is not economical due to only poor space-time yields, since more than ⅔ of the reaction volume consists of methanol and since the ethanol elimination, to suppress by-product formation, requires maintenance of a temperature of less than 100° C. At these temperatures, the ethanol, as reported explicitly by the cited reference, can be eliminated only gradually. If the reaction conditions are not controlled strictly, compounds which contain ether groups and have the structural unit Si—CH2—O—CH2—Si are formed under the acidic reaction conditions, and these contaminate the product. In addition, it is necessary to pass through an intermediate, in this case ethoxy(hydroxymethyl)-dimethylsilane, which constitutes an additional operating step. Moreover, Chemische Berichte 1966, vol. 99, p. 1368-1383 states that, after the distillative removal of the methyl acetate, a neutralization step is conducted with potassium hydroxide and CO2 before the actual product, 2,2,5,5-tetramethyl-2,5-disila-1,4-dioxane, is obtained, which additionally makes the process laborious.
The reference Organosilicon Chemistry, Scientific Communications, Prague, 1965, p. 120-124 shows basically the same reaction route in the form of reaction equations, but does not contain any working or procedural instructions which would enable one skilled in the art to comprehend the reaction sequence shown therein or to isolate a product. p-Toluenesulfonic acid is likewise named as a catalyst in this reference, which is why the problem of ether formation as a by-product in the case of this reaction route must inevitably likewise occur.
The siloxacycles 2,2,5,5-Tetramethyl-1,4-dioxa-2,5-disilacyclohexane, 2,5-dimethyl-2,5-diphenyl-1,4-dioxa-2,5-disilacyclohexane and 2,2,5,5-tetraphenyl-1,4-dioxa-2,5-disilacyclohexane were prepared by condensation of (hydroxymethyl)dimethylsilane, (hydroxymethyl)methylphenylsilane and (hydroxymethyl)diphenylsilane respectively, with elimination of hydrogen, as reported in Zeitschrift für Naturforschung B, 1983, vol. 38, p. 190-193. Since the reacting COH and SiH groups are in the same compound and hence are inseparable during reactant storage, the silicon hydride used as a reactant can, however, start this reaction at any time in an uncontrolled manner if it comes into contact, for example, with catalytic traces of bases. The preparation of any great amounts of silanes having both an Si—H group and a carbinol group is therefore very hazardous and can be implemented on the industrial scale only with a high level of complexity, if at all.
Moreover, the specialist literature describes various methods for transesterification of silanes bearing an acyloxyalkyl group.
DE 1 251 961 B describes the preparation of cyclic silane compounds whose structure can be represented by the formula *—O—R′—SiR″2—* where * is the point of ring closure and R′ is a divalent hydrocarbyl radical which connects the silicon and oxygen atoms via at least three carbon atoms. This involves subjecting an ester of the structure acyl-O—R′—SiR″2—OR′″ to a transesterification reaction with an alcohol. If the thus prepared compounds of the structure *—O—R′—SiR″2—* are reacted analogously to reaction I with silicone oils, the products formed, however, have a comparatively high organic component since R′ has at least three carbon atoms, which is disadvantageous with regard to properties such as flame retardancy of the successor products.
Union Carbide has described, in several applications (see EP 129 121 A1, EP 120 115 A1, EP 107 211 A2, EP 106 062 A2, EP 93 806 A1, EP 73 027 A2 and EP 49 155 A2), the preparation of acyclic products having repeat units of the structure *[O—R′—SiR″2—]p* (*=end groups or undefined groups). This involves subjecting an ester of the structure acyl-O—R′—SiR″2—OR′″ to a transesterification reaction with elimination of an ester acyl-OR′″, which is distilled out of the reaction mixture, the chain length distribution p of the product being controlled by the extent to which the transesterification is driven, and it is possible to add, as regulators to limit the extent of transesterification, high-boiling esters such as ethyl benzoate, methyl benzoate or ethyl laurate, which bring about blocking of the * end groups of the product by incorporating the acyl radical and the alkoxy radical of the high-boiling ester added into the product as * end groups. However, the preparation of cyclic compounds which could be isolated or purified, for example, by distillation has not been described.
The preparation of homocondensates of (hydroxymethyl)-silanes is also described in DE 44 07 437 A1. However, the document describes only how transesterification of (acyloxymethyl)silanes with alcohols gives an inhomogeneous mixture of linear or branched condensates.